1. Field of the Invention
The present invention relates to the control of high or medium power components of triac type.
2. Discussion of the Related Art
A triac is a switching component which corresponds to the antiparallel association of two thyristors. It is thus likely to block or to let through voltages of one biasing or the other, that is, it may be connected directly in an a.c. network, for example the mains.
A triac includes two main terminals A1 and A2, and a control terminal, called the gate. A triac is likely to enter conduction when a positive or negative potential is present between its terminals A1 and A2 and a negative or positive current pulse flows between the gate terminal and terminal A1 which will be called the reference terminal or main terminal associated with the gate. The triac then remains conductive until the current flowing through it transits through the zero point of the alternating current.
It is known that, when it is desired to control the conduction of a triac for a duration which is long with respect to the mains period, it is advantageous to turn it on at the time when the voltage across the triac is close to zero. This type of control is called a control at the voltage zero although, in fact, it occurs as the voltage across the triac is sufficient to enable its turning-on, for example, around a few volts.
This type of control has the advantage of reducing the current constraints upon switching and to reduce parasitics and harmonics that may be fed back onto the mains. As a result, the reliability of the triac increases. Further, this type of control is now imposed by various standards.
Several known circuits implement this function. However, these circuits exhibit at least one of the following disadvantages. They are complex, require a great number of components, some of which cannot be integrated, for example, capacitors of high value, and/or consume non-negligible power. Some of these circuits even require the use of an auxiliary supply source.